Wiki Word: Barycenter

Definition: The barycenter of an object is the center of that object's gravity, or mass.

      If you could rest a ruler on your finger, you would need to put the halfway mark exactly on tour finger. Because it balances on your finger at the halfway point, that would be its center of gravity, or barycenter. The barycenter is when the density on either side of an object is balanced equally. Therefore, if you tried to balance a sledge hammer on your finger, it would not be balanced because the side with the head on it is much more dense, and weighs more. This effect would send the hammer falling to the side of the head if you tried to balance it in the middle. 

       Athough you have heard that Earth revolves around the sun, this is not conpletely true. The Earth and the Sun revolve around each other, around the center of the mass. This is the barycenter. As I have already described to you, the mass of an object helps to determain the barycenter. Because the sun has more mass than the Earth, the barycenter of the two will be much closer to the sun than the Earth. Another example of this is the Earth and moon. Because the moon has less mass than the Earth the barycenter would be much closer to the Earth than the moon. The Barycenter is the center of an objects(') gravity. 

This term applies to our class because we are studying a new unit of outer space. We will most likely be using this term when we talk about the gravity and the revolutions of the planets of this solar system. We have also been talking about the reasons for the seasons and learning about revolutions and how the Earth's revolutions around the Sun are part of them.

http://spaceplace.nasa.gov/en/kids/barycntr.shtml

http://en.wikipedia.org/wiki/Center_of_mass

"Barycenter" (book) 

http://www.earthmatrix.com/moon/barycenter.html